Fig. 1. schwächling is a new allele of Arp3. (A-F) Embryonic muscle pattern of stage-16 embryos stained with anti-β3-tubulin. (A) Wild-type. (B) Unfused myoblasts are attaching to growing myotubes in schwächling homozygous-mutant embryos (arrow). (C,D) The deficiency Df(3L)ZP1 shows a similar phenotype to schwächling (C) and also displays an unfused-myoblast phenotype in transheterogeneity to schwächling (D). (E,F) The EP-element insertion Arp66B^EP3640 (E) only displays a fusion defect in embryos that are transheterozygous to schwächling (F, arrow). (G,H) In situ hybridizations with a labeled Arp3 antisense probe show that the Arp3 transcript is expressed in the mesoderm in stage 10 (G) and late stage 11 (H). (I) RT-PCR on RNA isolated from wild-type, Arp3^{schwächling–/+} and Arp3^{schwächling–/–} embryos. Spliced Arp3 mRNA is present in all three lanes (509-bp product). The zygotically transcribed mRNA in Arp3^{schwächling–/+} and Arp3^{schwächling–/–} embryos is unspliced (674 bp). (J) Schematic drawing of the Arp3 transcript. The ORF is denoted with shaded boxes. The donor/acceptor splice-site nucleotide sequence of intron 1 from wild-type embryos and Arp3^schwächling mutant embryos is shown. schwächling mutants show a G-to-A transition in the acceptor splice site.